The Germans had been tricked by the Allies thinking the assault would come at Dunkirk. They have encountered fake paratroopers, called "Rupert" by the English, which set of fire crakers when it hit the ground. The Germans, thinking it is paratroopers firing at them start shooting the fake paratroopers. When the High command in France heard of paratrooper landings in Normandy, the assumed that the paratroopers were a fake diversion for them to move there Panzer division to Normandy. Hitler ordered The Panzers to stay at Dunkirk because it is the closest to England. When the Panzers were finally called in, it was too late.